1.获取图片的原始宽高，并获取到图片原始宽高的宽高比

let src = "图片地址"
wx.getImageInfo({
    src,
    success: function(res){
        let originalWidth = res.width;
        let originalHeight = res.height;
        let originalScale = parseInt(originalHeight/originalWidth);//图片高宽比   
    }
}）

2.获取屏幕的宽高，并获取到屏幕宽高的宽高比

        wx.getSystemInfo({
            success: function (res) {
              let windowWidth = res.windowWidth - 36 ;
              let windowHeight = res.windowHeight;
              let windowscale = parseInt(windowHeight/windowWidth);//屏幕高宽比
            }
        }）

3.计算图片尺寸

if(originalScale < windowscale){//图片高宽比小于屏幕高宽比
    //图片缩放后的宽为屏幕宽
    imageSize.imageWidth = parseInt(windowWidth);
    imageSize.imageHeight = parseInt((windowWidth * originalHeight) / o